In The Walking Dead, the survivors have seen plenty a wall. From Woodsbury, to Wellington, to Alexandrea, they are all different and they all have their advantages. One issue that none of them have managed to fulfill yet is the gate though.
Every wall in the walking dead, and in history, has one major weakness, the gate. No matter how strong the wall is, the entrance to that wall is weak.

Take for example, Alexandria. Their wall is made of large metal plates held together with steel supports and mound of dirt behind them displaced from a frontal moat. While this wall is successful at repelling any threat, when thousands of zombies came knocking at their door, the door fell off its hinges and the wall became useless.

This made me realize that the gate is the primary weakness of all walls and also made me wonder; is there a gate design that can repel hundreds, if not thousands of zombies, while still allowing humans to enter?

$\begingroup$A decent steel door will stop them, they are zombies not superman. They are not any stronger than people. If you really want to overbuild it look at blast door designs.$\endgroup$
– JohnFeb 22 '17 at 15:06

28 Answers
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We humans are pretty good at building gates to keep people out. We've been doing it for a long time for military purposes and to stop thieves.

Why Do We Have Gates?

Because there has to be some way for friendlies to get through the defenses while keeping the bad guys out.

From a military perspective, a gate isn't there to stop people from getting in. They'll get in. Instead, it's there to channel them into a particular area by the allure of an easier passage. Avoid the wall and its defenses, drive through the gate! In anticipation, that particular area is set up as a kill zone: a deadly crossfire will be in place to mow down anyone who comes through, combined with removable obstacles to slow them down and pin them in the kill zone.

From a security perspective, a gate is also there to slow down a thief. But instead of a kill zone, it exists to be such a nuisance to force the thief to give up, or to make so much noise getting through they're detected. Once detected, forces are rushed in.

So, gates exist to...

provide access through defenses for friendlies

herd attackers into a kill zone

force the attacker to reveal themselves

slow the attacker to there's more time to deal with them

Zombies don't act like an intelligent, coordinated military force, nor do they act like thieves. They just walk, slowly, towards food. They might not even find the gate and instead just walk into the wall in whatever direction they came from. (Note: I only watched the first few episodes of The Walking Dead, so I don't know exactly how their zombies act).

That is until you start making a bunch of noise going in and out of the gate. Then they'll go investigate. The problem is not having a big rush of zombies at the gate, the problem is having a slowly growing, constant pile up at the gate which is putting more and more pressure on it until, like a fence at a football riot, it will collapse.

Zombies also don't get tired and give up. They'll keep banging on that gate and walls until one of them breaks.

This means you need an active defense to clear the zombies off, and you need to do it efficiently and safely.

Establish A Kill Zone

Take advantage of that pile up and use it as a kill zone. The zombies will be easy pickings as they pile up against the gate, or even the walls. A few isn't a problem, unless you need to go out. Take care of them before there's too many.

Kill Them Cheaply

You want to set up your gate and defenses so the zombies can be killed without expending resources. This means no guns and no gasoline. There's going to be a lot of them, and you only have so many bullets. Save them for emergencies and foraging.

Kill Them Safely

You're going to be doing this a lot and even a single bite means you're dead. That means it has to be done very safely, no heroics.

Bars + Pikes

The simplest actively defended anti-zombie gate is some sturdy iron bars and a few people with pikes. Zombies thrash against the iron bars, the pikemen stick them in the face from well outside of arms reach. Cover the end of the shaft in grease to prevent the zombies from grabbing it. If they do, pikes are cheap. Let go and grab another one. Maybe you can get the lost one later when they're all dead.

These bars can and should be placed all around your defenses to prevent zombies from piling up at any point on the wall.

Flexible Gates

Against an enemy like a zombie, rigid defenses might not be the best idea. Instead, you might want to try a [flexible security gate] like you'd find in a mall.

Unlike rigid bars which will take all the force of zombies banging on it, a flexible security gate will flex and wobble which will absorb some of the impact. And they have those handy slots for stabbing zombies through.

Just make sure it's strong enough to hold back a horde pressing against it, and it's secured to something that's strong enough, too.

Evolution 1: The Murder Hole

This is, essentially, an inner and outer portcullis: a big, heavy gate with spikes on the bottom that drops from a height.

Now you can better manage the number of zombies you're dealing with at once. Once you're done, you can get into the inner area to clean the bodies up and avoid a pile up.

There's any number of improvements which could be made to this, for example a grilled platform on top, out of arm's reach, to stab down through.

Should the inner gate fall, you only have to deal with a limited number of zombies inside the wall. Should the outer gate fail, you have an inner gate to fall back to. This brings us to our next point.

Have A Mobile Backup Gate

Taking a page from The Road Warrior, use a heavy vehicle like a bus as a mobile gate. But instead of using it as your primary gate, have it ready as a mobile backup gate to close any breeches in the wall. Just drive it up to block the gap. This will plug the breech and give the defenders time to react.

Evolution 2: Dig A Pit, Fill It With Spikes

If the Walking Dead zombies are like the Romero and Max Brooks zombies, they aren't very bright and will happily walk off a cliff if there's food on the other side.

To make our murder hole even safer, we can add a pit filled with spikes.

Raise the outer gate, zombies fall into the pit and get impaled. It probably won't kill them, but it will immobilize them. Then stab them with pikes at your leisure. You can even throw a grating over the top to make the stabbing even easier.

Evolution 3: The Crushinator

You've got these zombies pressed up against your gate, you know where they're going to be, why not use it to your advantage? Scrape them off the gate with a heavy object.

|
O|
-------|v
| |
| |Z
===================

You have a heavy door (optionally sharp) held above and just outside the gate. Zombies press against the outer gate. Drop the door right on their heads. Crush the zombies. Lift it again. Repeat until all zombies are dead or disabled. Finish them off with pikes. Safe. Efficient. And it's a spare gate!

Don't Have A Gate

Taking another page from the original Dawn Of The Dead, do you really need a gate? What about a ladder or ramp? Zombie attack? Retract the ladder. Just be sure you have a way of clearing the zombies off the entrance, like a set of bars to stab them through.

7 Days To Die is a "survival, horde, crafting game". It's Minecraft meets Left4Dead. It's a great place to try out various anti-zombie defense schemes.

What sets 7D2D apart from other zombie survival games is everything is destructible. Wood, metal, concrete, even the ground itself, the zombies will chew through it all. Any passive defense will eventually be ground down and fail. I've had zombies chew through concrete, wriggle through firing ports, climb over each other like ants, tear the foundations out of platforms, and even come up through the floor!

The other thing that sets 7D2D apart is right in the title: every 7 days, ready or not, you will be attacked by a horde. It lends a certain time pressure to everything you do. Grand plans give way to necessity. Are you going to try to explore to find the mixings for concrete and bullets? Or chop down more trees to bodge together more spikes and arrows?

Finally, while basic combat is rather simple, getting hit is brutal. Especially at the higher difficulty levels, one hit and you're stunned, two or three and you're dead.

The game is still in early access, but it's totally playable and totally brutal. Big changes are coming in Alpha 16 in March, so you might want to wait until then. But if you want to torture test your ideas about zombie defenses, give it a shot.

Here's the aftermath of a horde night in my fortified chateau. It took the "no gate" philosophy. All stairs were torn out, even interior stairs, and replaced with ladders and armored hatches. We attracted them to a particularly well fortified corner of the house.

That was four layers of spikes. Note the firing ports for an active defense. Also note the hasty repairs on the concrete; they got through, but that was just the outer defenses. We had multiple backups to retreat to.

$\begingroup$7DTD is a fun game, but it has very little to offer in terms of physical realism, and your building options are very limited as well. Those flexible gates, for example, you absolutely cannot create. And a zombie can crush through 1 meter thick wall made of reinforced concrete with its bare hands, overnight. It's nothing like "torture testing your ideas".$\endgroup$
– HeadcrabFeb 22 '17 at 3:57

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$\begingroup$@Schwern Yet in real life you can sit behind indestructible walls all you want, unless the enemies have special equipment (siege machines, explosives, artillery, all that stuff zombies never use), and sometimes even if they do have those. 7D2D presents things differently for the sake of entertainment (same as TWD does), but I don't think the original question was about game mechanics.$\endgroup$
– HeadcrabFeb 22 '17 at 6:34

$\begingroup$If cheap killing is the goal, why not have rotating grinders in place of a gate? Power them by putting treadmills outside, thus the zombies themselves provide most of the energy needed to grind them to bits.$\endgroup$
– Hugh MeyersFeb 23 '17 at 11:16

Human engineering can, 100%, keep out a horde of zombies. In The Walking Dead things fail because if they didn't the show would be boring. In real life, there exist castle gates hundreds of years old which would hold, no questions asked.

First and foremost, realize that thousands of zombies will never be pushing on the gate all at the same time. They will spread out against the wall as the pressure mounts from behind. Thus, the gate doesn't need to be nearly as strong as you'd think.

Second, take a pro tip from me, and don't build your gate on wheels, or out of chain link fence, as is the case in most walking dead situations. A draw bridge can be built. A portcullis, perhaps. These things are not rocket science, and will work much better than half the crap you see on that show.

Bracing the gate would also be a good idea. Move a bus against it, or drive large stakes into the ground at intervals. Check out the siege scene in LOTR when helm's deep is being assaulted. Watch the soldiers barricade the gate. Now that's an attempt at keeping the enemy out, not the half baked efforts in TWD (if they weren't constantly being defeated and driven out of their "fortresses" the show would have been over in Season 3).

Since you're asking for a design of a gate, here's mine:

A gate you push into place from the side - large metal one (but not too long, as the longer it is, the more difficult it becomes to brace it). I'm thinking steel plates welded onto a nice, thick frame. Because you don't just want it to get pushed over, you need to secure it very well at both ends. However, that might not prove sufficient in a "million zombie horde" situation.

You have to have the option of bracing it from the inside such that it doesn't buckle inward, and come free of one of the end points (and in order to keep some pressure off the end-points). Have stake pits dug, and brace the gate both with those, as well as with heavy beams.

And if you really want to survive, have the option of lowering a plow -like device over the front of it.

Imagine a giant horde coming your way. You seal the gate, put in the stakes brace it, and then swing a big metal "V" down over the side, such that zombies coming up against the gate simply push each other to the side, and into the walls.

Future survivors will come across your fortress years later, see the tall, proud gate still standing firm, and think:

"Wow, that sure is a great gate. Too bad they didn't build the walls as strong!"

$\begingroup$Definitely love this answer. TWD does a lot of things that logically make no sense. IRL mobs will trample each other, crush each other against walls. A herd of zombies against a wall wouldnt make the wall fall, they would pulverize themselves. Even a very well supported chain link fence might just result in julienned zombies, not the fence coming down.$\endgroup$
– theinvisibleduckFeb 22 '17 at 16:13

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$\begingroup$@theinvisibleduck - chain link fence is not meant to hold a hundred "people" pushing into it, however some of the fortifications they built should have held. The show contrives reasons to make those fortifications fail, such as a guard failing to see a horde of zombies, or some !d!ot leaving the gate open (who the f*** does that in a zombie apocalypse?!?!) The amount of stupidity in that show is ultimately why I stopped watching it. Ninja zombies teleporting behind people, characters slaughtering zombies by the dozens one moment, yet being overwhelmed by a single walker the next, etc.$\endgroup$
– AndreiROMFeb 22 '17 at 16:19

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$\begingroup$I'm surprised nobody has mentioned the strongest door of all; a bank vault door. They use a similar design to uPVC doors, with the metal moving into place around the frame when it's locked. Extremely strong design. I've seen police raids where they couldn't get through a uPVC door.$\endgroup$
– n00dlesFeb 23 '17 at 7:16

Use the power of physics, dirt, and the stupidity of zombies. Zombies break a door by exerting force against it en masse, but the zombies have to have an equal and opposite source resistance to push off from. Therefore, you give them extremely limited terrain to push off against the gate and then give the main horde something insanely difficult to deal with, like 15 tons of packed dirt with a concrete shell.

It doesn't matter how big the horde is - they'll never break a gate that's situated like that, since the horde isn't capable of putting more than a few zombies in position to actually put pressure on the gate.

It is possible that the zombies could dig through, assuming you just have dirt and given enough time, so you could either commit to clearing them out occasionally or use at least an outer shell of concrete or something. The zombie blood should make it impossible for plants to grow (too alkaline,) so you won't even have to worry much about plants weakening your concrete.

Now, you may be wondering how this is different than a regular moat, as has been suggested in several answers. The key difference is that is takes an astronomically huge number of zombies to fill this 'moat', provided that you make the solid section steep enough to prevent the zombies from stacking on each other with the ability to push against each other. Your 'moat' is either the entire valley (if you're in a valley) or the entire planet below your elevation (if you're not in a valley.) Now that's, one heckuva moat.

Finally, I realize that this isn't really putting much effort/thought into the gate itself, but (as in many things,) it's all about location, location, location.

Don't let them even touch your door.

You have your nice tall walls, you also have your nice gate behind a drawbridge and moat.

But this is a horde of the living dead we're talking about!

Moats can end up filled with the writhing mass of the living dead who can eventually climb over each other to scale your wall or batter down your door.

Water doesn't cut it.

The solution.

Bellow and in front of your door you place a rock crusher salvaged from a quarry. You set up your fortifications next to a large fast flowing river to provide power for your crusher.

The zombie mulch can also be ejected into the fast-flowing river to make sure you don't end up with a pile of rotting zombie flesh.

This keeps the population of zombies around you walls low: Zombies are attracted by the sound of the crusher and any who stray into the crusher get pulped. When you want to let humans in you lower your drawbridge to make a path over the crusher.

$\begingroup$I would imagine that overtime the rock crusher would be prone to mechanical failure.$\endgroup$
– John TFeb 23 '17 at 2:57

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$\begingroup$@JohnT given they're designed to crush solid rock into gravel I suspect that it would require less maintenance than the same machine in a working quarry.$\endgroup$
– MurphyFeb 23 '17 at 11:47

$\begingroup$@Murphy In terms of physical wear, yes. But blood is salty and corrosive... Remake all the bits you can out of stainless steel and be sure to grease the bearings after every use I guess...$\endgroup$
– PerkinsFeb 23 '17 at 19:45

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$\begingroup$A replacement-parts supply would be critical, as would fallbacks in case the crusher broke, slowed down, lost power, etc. If you're talking about the LONG long haul, the psychological angle would be important as well: everything runs down, and if the residents of your fortifications came to depend on/take for granted the crusher's continued protection, on the day that it eventually did break down, things would get very dire.$\endgroup$
– Zac BFeb 23 '17 at 22:44

As Andrei has already pointed out, if the doors didn't fail, there wouldn't be much of a series. However there are many imaginative ways to deal with zombie hordes if you have time to prepare. Here are a few.

Why have a door?

Just have a solid wall all the way round and a ladder. Zombies aren't known for their intelligence, when under attack remove the ladder, there's no door to fail.

I really want a door though

Interlocking doors that only open outwards. Vault doors are designed as a conical section, they will only open outwards, the more you push the tighter they fit. If you want a pair of doors, then overlapping panels such that again, the door only opens outwards. Like these lids, but bigger:

That's all very well but, I'm a horrible person

What you need is a sharpened portcullis. The pressure from the zombies behind push the ones at the front through what is effectively a large scale potato chip maker (french fry cutter). You don't even need to kill them yourself, just have a pit for the bits of zombie to land in. Severing the head is irrelevant on a neatly cubed zombie.
You may want a blade you can drop down the outside to clear away the buildup of bits of zombie on the cutting edge.

$\begingroup$@Separatrix wood chippers are too prone to failure since they're only designed to process wood, so zombie chunks and bones might gum it up faster. Use a scrap shredder instead (youtu.be/3uomoLK97og) :D$\endgroup$
– Doktor JFeb 23 '17 at 23:10

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$\begingroup$Another idea, if you go door-less, is to have rope with a loop at the end and a knot about 4-5 feet above that. If someone (known human) is approaching the wall and needs a quick way up, you can throw the rope to them; they put their foot in the loop and hold the knot, then you pull them up. In contrast to a ladder which has to stay on the ground until they've reached the top, as soon as they've hooked their foot and grabbed the knot you can pull it and there's nothing left for zombies to climb.$\endgroup$
– Doktor JFeb 27 '17 at 18:54

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$\begingroup$If the poor sap doesn't make it in time and the zombies grab their leg and pull them down, the rope can be released from the top so once more, there's nothing left for the zombies to climb. If the rope is still intact after the zombies have left, it can be recovered (and cleaned... eww) later.$\endgroup$
– Doktor JFeb 27 '17 at 18:55

From an engineering perspective (and having done mechanical engineering for better than 30-years, with a few college degrees for the trouble), I can say that the problem with doors (in reality) isn't that they fail, per-se, but that their attachment points usually fail. The 'threshhold' (door frame, hinges, latching hardware, and subsequent structure framing) are actually what fail most of the time, when dealing with real-world 'doors on old buildings'. Add to this problem the issue of a mob-horde pushing en-masse into the door, and you have a 'maximum material condition' best addressed by material properties considering age.

To bring this down to a layperson's perspective, a 2-inch thick oak door, itself, isn't going to fail an interior hatchway (where water or weather aren't a significant aging factor). Failure will come from mechanical stresses (aging, fracturing, etc.) of the mounting hardware (all iron-based metals fracture, in-time).

Were I building a 'thousand-year-zombie-door' (in absolute theory, here), I would start with a granite double wall set up for a pocket-door (where the door slides, but not as a sliding glass door does - a pocket door is 'hidden' in a wall, when opened via being slid back). I would ensure that the header and footer track were at LEAST 15-percent the height of the passageway itself (so the door would need to be 30-percent taller than the opening itself, as it would also need to be at LEAST 30-percent wider). I would use brass or stone rollers on the top rail, and a secondary grooved channel rail at the bottom (to allow for drainage and prevention of the door galling into an immovable state over time). For the door material itself, I would most-likely go with either 45-55 mm thick stainless steel or high-phosphor bronze plate (so the door would most-likely weight a couple of tons).

This would then provide for thousands and thousands of kilograms of resistive force for centuries, without reaching material failure points. It also allows you to still slide the door open with thousands of zombies on the other side, otherwise blocking you from opening it later, when you want to get out. Remember one thing, as they pile in, and block the door, they are also still 'juicy' and will rot an organic door and jam any fragile hinge, latch or roller mechanism. No jewelry box doors here, think more like aztec pyramid construction methods instead.

You now have a worthwhile fort door that can be opened with a simple pry lever, at worst, that will last a millennium, and still look good being a 'door'.

Oh, and you might want to remember a simple rule of combat: Make a fortress too hard to get into, and you'll build the perfect crypt you can't get out of!

Sure there is, it's been done already. A lot. Much field-testing has been done on the best way to break through a large door, especially when considering that the defenders will likely not be pleased that it's happening, although the tests were performed a while ago - a few thousand years ago, to be exact, when castles were still in fashion and a rival lord's raiding of it not out of the question. The solution for the attacker was a large battering ram, or to skip attacking the door entirely and just demolish the castle with siege weapons. Zombies do not have the capability to carry a battering ram, however, nor operate siege weapons. And those rams were only meant to bash through wooden doors. Doors of stone or even metal would be damn near impenetrable to human - or zombie - strength.

As to why it didn't work on the show...well, it's on TV, and I blame the fact that the writers just wanted ~~DRAMA~~ and cared more about that than physics.

$\begingroup$No, mythbusters tested it, the combined strength of only 25 people was enough to not even be slowed by a wooden door, never mind hundreds. Never mind over a long period of time$\endgroup$
– TrEs-2bFeb 21 '17 at 20:45

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$\begingroup$@TrEs-2b Really? I never saw that episode. Also, were they testing the front door of someone's house, or a massive slab of oak wood, well over a foot thick, and held together with iron bands and screws? Because that's the kind of "front door" people put on castles.$\endgroup$
– JacobFeb 21 '17 at 20:48

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$\begingroup$@Jacob They were testing a regular barn door, if I remember correctly - probably stronger than some doors made of glass and plywood, but still nothing like castle gates.$\endgroup$
– HeadcrabFeb 22 '17 at 4:05

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$\begingroup$@TrEs-2b when hundreds of unrelenting bodies combined are pushing against the door, the first ranks get quickly squashed into bloody pulp, unable to keep pushing, then the ranks behind them, etc. Even if you're an animated corpse feeling no pain, you still need some structural integrity of your body to be able to exert force.$\endgroup$
– HeadcrabFeb 22 '17 at 4:12

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$\begingroup$The middle ages were more like hundreds of years ago than thousands of years ago, though. It is usually defined from somewhere in the 5th century to somewhere in the 15th, so between 1500 and 500 years ago.$\endgroup$
– GraipherFeb 22 '17 at 9:02

Something to keep in mind: Zombies can't develop any huge pressure against the door. Their bodies aren't able to take infinite pressure, if enough of them are trying to get through your door the ones in front will be crushed and cease to be able to provide pressure. Thus sufficient zombies means that the pressure actually is reduced, not increased.

Besides that, there's a simple way to build an entryway that will stop a horde of zombies. You need a moat, ideally with a river flowing through it. Unlike a normal castle you want the moat inside your walls. Build your outer wall to funnel the zombies into your gates. Your gates extend across the moat except the floor is actually hinged to the bottom of one wall and counterweighted with 300# more than needed to balance.

A person walks across them, no problem. The walk up to the door, slide it to the side and enter.

The walls and door are tough enough to take a few zombies. Once 300# of zombies accumulate in the corridor the floor tips and they're dropped into the moat. Obstacles are arranged to ensure they die on the way down. Then the floor rotates back into position and more zombies advance to their death.

If you have a river in your moat the corpses go off to sea rather than pile up underneath (a sufficient pile would jam the door.)

$\begingroup$Pressure is also reduced because the gore of the pulverized zombies will act as a lubricant, making it more difficult for subsequent zombies to actually gain traction against the gate as they slip around in the muck on the ground and slide back and forth against the door rather than pushing into it.$\endgroup$
– Doktor JFeb 23 '17 at 23:14

$\begingroup$@DoktorJ No, the zombies fall, they don't mess up the door. It doesn't matter, though, enough weight and down they go.$\endgroup$
– Loren PechtelFeb 24 '17 at 3:52

$\begingroup$I was referring to the first paragraph alone (if someone didn't have the technical acumen to build a mechanism like you're referring to). In this case, the zombie mush would accumulate, but as I said would act as a lubricant and actually help :P$\endgroup$
– Doktor JFeb 24 '17 at 17:04

$\begingroup$@DoktorJ Oh, I see, you were addressing my point about the pressure rather than my solution to the zombie proof door. Agreed.$\endgroup$
– Loren PechtelFeb 25 '17 at 2:29

I don't know if a gate can stand the force of hundreds of (undead) humans, but there is indeed at least one way of making the gate almost as strong as the walls around it.

a- Use a sliding/rolling gate instead of hinged design

The gate slides up and down in the wall. Its edges are 3 feet inside the wall at both ends. There are no hinges anywhere, just ball bearings, wheels and a high friction surface (of the walls) for the wheels to roll on.

b- Use a few long, thick spikes on the gate

This will help hinder the zombies from pushing at the door altogether. Considering that the zombie who tried to push in, gets spiked and unable to move, it will be very hard for the zombies behind that first zombie to shove him aside and make their way forward.

$\begingroup$This is part of the appeal of a drawbridge or Portcullis. There are no hinges or latch to break down...you have to literally demolish the door or the walls in order to get past it. Much harder than breaking the hardware on the door.$\endgroup$
– guildsbountyFeb 21 '17 at 21:46

$\begingroup$Bonus: highest concentration of spikes at head level -- go for the brain!$\endgroup$
– Doktor JFeb 23 '17 at 23:11

A gate that comes down from above. For example, stairs that fold out from the side of an aeroplane or a rope ladder lowered from a kid's treehouse.

Zombies are dumb. They won't climb, just mill around.

Build a wall with no gaps, and have a crane raising and lowering a large platform, big enough to fit a truck. You could make some noise on one side, attracting all the zombies to that part of the wall, and then drive the crane to the other side of the compound and its platform without having to worry about zombies piling on.

A door which can only open outwards, on rails.

The black part is the wall. The gray part is the "door", made of the same material as the wall.

If you want to open the door you must slide it on the blue rails outwards.

When zombies arrive, they could

try to push the door, which will then push against the walls, effectively becoming part of the wall. They will not get in.

(assuming some intelligence) realize that the door moves outwards and try to pull out.
The red part comes then into play: it is solidly attached to the door and turns horizontally to push against the walls (when one wants to prevent the door to be open (=pulled outside)) or vertically in normal times. There could be other designs here, the point is to make sure that one cannot pull the door out from outside (a hinge could be a better choice)

Don't discount the moat/drawbridge. If the moat is right up against the plane of the gate, it will have the following effect. The shambling dead come, fall in the moat, more of them come and fall in eventually filling it up. Here is the thing, piles of squirming, hungry ghouls will not make for good enough footing for the remainder to be able to generate tons of pressure against the gate.

In addition, a gate with two doors, that is well anchored against a solid wall, can be built to swing out when open and when closed the 2 sides will overlap and lock in place, with any pressure binding it more solidly against the framework.

I'm trying to work in ChaosCenturian's Zombie cheese cutter as well, but not having a great amount of luck.

I'm thinking of a beefed up revolving door, the type you see at the mall to lock the air temperature in.

Then build a mechanism, an additional blockage on the inside, that can either open inward, or seal the way in.

So to open for friendlies we can keep the channel open through. Humans can push the door half a cirle and walk in like normal. Comes a zombie and we close the plate inside to block the passage, and he can spin the door the whole circle just to end up right back outside, wondering in a zombie way what is going on...

And better yet, build that whole gate as a turbine. Make some noise to invite thousands of them to come and spin the gate to generate some electricity for us...

$\begingroup$Hello and welcome to WorldBuilding.SE! I think you can leave the part about this being a joke out. An answer should always aim at providing an acutally helpful answer to the question that will help OP and other readers. And I don't think this is just a joke-answer. Using zombies in creative ways to make electricity is alwasy interesting and using a revolving door is something most of us probably haven't thought about. There are some problems I see. E.g. what happens if the zombies try to push in both directions? And how will the humans use this if there is horde behind them?$\endgroup$
– SecespitusFeb 22 '17 at 22:21

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$\begingroup$Welcome to WorldBuilding! I edited your answer to remove the "this is joking" comment. Your answer seems like a reasonable answer, even if it isn't the best answer on the question, so no need for the joking tag. Besides, if it was just a joke, then it would be subject to being removed. :-)$\endgroup$
– SRMFeb 22 '17 at 22:21

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$\begingroup$@Secespitus Great minds think alike. :-) In the future, don't be afraid to make edits like that to improve answers. The edits can be rolled back if you go too far.$\endgroup$
– SRMFeb 22 '17 at 22:22

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$\begingroup$@SRM Thanks, but I don't have the reputation yet to edit questions/answers single-handedly. They would need to be reviewed, which often takes some time. Therefore it is faster for me to write a comment and point it out to OP so that he can change it.$\endgroup$
– SecespitusFeb 22 '17 at 22:25

$\begingroup$In one of the The Walking Dead episodes, there is such a situation with revolving doors and zombies, where the characters get caught inside of it while zombies are pushing on both sides. It doesn't end well, although it's due to the fact that there are zombies on both sides of the door.$\endgroup$
– AdrienFeb 27 '17 at 11:34

Worried about many zombies being at your gate and it weakening over time due to too many zombies pushing on it.

You need to keep them split up.

Barriers are used for this exact purpose during festivals, the idea is to limit the amount of pressure on the front of the crowd zombies.

For example:

The zombies are kept back from the main stage using barriers, there is another set just out the camera view preventing crush events. Using this we can lower the pressure on the gate, so the gate doesn't need to be as massively built.

Spiting them up can make it easier for the other methods to deal with the zombies. Combined with other methods this can be effective as well.

A Drawbridge:
Raised, say, 5m above the ground level of the rest of the wall, so that it opens on the second or third floor. The other side lands on a raised cantilever, like a viewing platform, at the end of a ramp. When raised, there is nothing connecting the two ends, so the the gate is inaccessible from the ground. When lowered, the bridge connects both ends and, depending on the material, you can drive heavy vehicles across.
Since from the ground level there is nothing different about the gate area, compared to the rest of the wall, the gate is secure.

$\begingroup$What prevents the zombie horde from simply filling up the moat with their own bodies, or for that matter piling up to the platform level? Even ants do that.$\endgroup$
– OmegacronMar 28 '17 at 20:08

1

$\begingroup$@Omegacron: That would imply intelligence on the zombies' part, and is a completely different problem. What I'm doing here is betting on the fact that zombies aren't smart enough to look up and thus only see uniform wall. Even when a transport enters or leaves, once the drawbridge is closed, zombies lose interest in the cantilever, other than walking along the ramp and falling down the raised end.$\endgroup$
– nzamanApr 1 '17 at 9:52

$\begingroup$As stated elsewhere, I always assume zombies have at least dog-level intelligence, just to be safe. But fair enough, I'll give you a +1 for the drawbridge concept. After all, it usually works for humans.$\endgroup$
– OmegacronApr 3 '17 at 17:59

You can build an indestructible wall or gate; at least, indestructible by meat.

The real problem is dealing with (say) 30 million corpses as the infinite wave of zombies crush against your base and die, forming a ramp of crushed flesh which the next wave crawls over.

A 100 kg corpse with the density of water has a volume of 100 L. 30 million of them comes to 3 million cubic meters.

Assuming a cone with a 10% gradient to the pile of zombies, the volume of a cone is $\pi r^2 \frac{h}{3}$. A 10% gradient means $r = 10h$, giving us

$3000000 m^3 = \pi \frac{100}{3} h^3$

which is roughly 30 meters. Now, a half-cone has half that volume, so to build a fortress capable of surviving 5% of North America's entire zombie population smashing against it and forming a human ramp, you need walls (and doors) that are roughly 30 meters tall, and the wall needs to be able to support a huge mass of zombies (roughly equivalent to water) on the other side.

30 meters of water generates about 3 atmospheres of pressure on the base of the wall, or 30 tonnes of pressure per square meter. 30 meters is about 4 stories.

The wall height needs to be tweaked based off of zombie pile slope and percent of continental population of zombies we have to survive.

The compressive strength of any reasonably built concrete is well beyond this; but any jury-rigged walls or doors are going to fail.

The door itself is a relatively small problem. Arrange it so that you cannot get a "run up" on the door by having the wall "wrap around" the entrance area with a choke point. Have multiple doors; I would go with 3, with only 1 every open at a time. An attack overwealming the outer door (a bomb by humans, or the door is accidentally unlocked) leaves at least 1 door sealed.

This follows the rules that you have a never event (zombies getting in), you need for an unprecidented failure to still leave you with a layer of safety.

Outer door fails while an inner door is open; no problem. Only lose people within the killing zone.

Here is a gate designed to surive against zombies. We have an outer wall that prevents "direct charge" of any of the doors.

Kill Zone 1 is an isolated area where zombies breaking in can be slaughtered from above/through holes in walls. If there are "low" places to fight from KZ1, access is via ladder into KZ1; you cannot flee from KZ1 out of it directly until zombies in KZ1 are dead.

The ability to "drop down" into KZ1 from the 30 meter (or however tall) walls above is possible, but this should never be deployed, and any drop-down should be from KZ2.

KZ2 and KZ3 works similarly, but it is behind the door of #, and are isolated from the safe inside by another door of #. One-way entry to the "killing locations" in KZ2 can be reached from KZ3, and KZ3 has "one way entry" to the killing locations from the inside.

The "killing locations" are things like raised platforms or iron-bar fortified ground locations that you can use pikes to fight from. These are used to clear the killing zones if besieged. These killing locations are not presumed safe from being overrun.

Doors should be things like thick metal bars with thin gaps lowered from above. Simply too massive to be dislodged by mere meat, and mere meat cannot stop them from lowering. Wooden doors that can be closed to block line of sight can be added, but are not a requirement for the barrier to work. Killing zombies through these doors should be possible.

Someone getting a truck and smashing the outer gate while the second gate is open is the "worst case" this is designed to defend from. Cinematic failure would consist of the people in killing zone 3 "killing location" fleeing, convincing people outside of it to lower rope ladders or somesuch, then being overrun and zombies getting inside.

So the next stage is dealing with exactly that; KS3's "one way" entrance needs to be "airlocked" in a way that doesn't permit local zombie infection from getting out.

Simple. A trapdoor. It requires being lifted up thus any zombies trying to open would have to bend down and grasp the door assuming they had the intelligence to do so. Even if smart enough the number of zombies lifting is limited to the number of zombies that can reach the door at one time.

If the trap door has no handles on the outside no zombies can even try to lift it.

In all likelihood the zombies would shuffle across the top and not even pay attention to it

$\begingroup$My general rule of thumb is that zombies are at least as smart as dogs. Anything a dog can get open, a zombie could, too. And given that my dogs can somehow get a locked trash can lid open to tear open the dog food bag, I wouldn't trust my life to a trap door with no handle. I'll still give you a +1 for the simplicity of the idea, though.$\endgroup$
– OmegacronMar 28 '17 at 20:05

From a purely defensive viewpoint, the best possible design for a gate or door is the same design that many bank vaults use. You take a heavy metal door which is smaller on the inside than the outside, like a ziggurat on its side, then put it in a matching door frame. The larger portion of the door prevents the door from pushing inward past the frame. Combine this with heavy hinges and pistons which slide out of the door into the frame, and you have a virtually unbreakable door.

This same basic principle is used in many military installations, such as the famous "red door" of the Cheyenne Mountain Complex in Colorado. In fact, the design is often called a "blast door" because it can withstand MASSIVE amounts of pressure from the outside. The weight of the door itself isn't really an issue, either - as long as the door, hinges, and surrounding wall are strong enough to withstand the amount of pressure exerted against it, a door of this design will NEVER give or fail.

Now, all that said... this is serious overkill for zombies. This door will hold even if the zombies are running into it with bulldozers at high speed, or ramming into it with tanker trucks full of gas, or even shooting low-yield nuclear weapons at it. And if they're doing all that, you're pretty much screwed anyway. Just accept your fate and die with some dignity.

Any reasonably heavy metal door or hatch, that closes into a tight-fitting heavy metal doorframe with minimal protruding edges, and which is equipped with several reinforced bolts and hinges, should be easily resistant to zombies (unless the zombies can figure out saws, explosives, or particularly effective battering rams).

(Think of the heavy metal lockers they have in campsites in bear country. Bears can rip into cars with their claws, because cars have windows and are made of thin sheet metal. These lockers are made of steel that is at a minimum 6mm thick, and reinforced.)

To resist battering rams better, make it sloped, and also grease the outside of the door and the surrounding wall so that zombies have a hard time grabbing it.

Also, as others have said, you're going to want to be able to shoot at the zombies attacking the door, defending it will make it much more resistant.

If I had the required time and materials, I would make a hatch out of 12mm thick steel, tilted at a 45 degree angle, reinforced with welded metal on the back, opening upwards, and seating in a doorframe made of welded 12mm-thick-wall box channel beam or angle steel. The "hinge" would be a ~16mm dia or thicker steel pin. Counterweights would allow a human to open it.

I can't imagine zombies defeating that; there's a limit to what brute force -- that still relies on human muscles -- can do.

As already pointed out, there are many ways from medieval times to build a zombie proof entrance. The problem is to build it.

Remember, we don't talk about medieval times, when you had a lot of time to prepare for a coordinated attack every so many years. We are talking about building your defenses under the constant thread of uncoordinated and mostly small attacks. So you do not want to build a complex structure which requires you to live with a hole in your defense for months, weeks or even days. Even a few hours can proof fatal.

We probably also want an entrance that is not by default open and can quickly be closed, like in medieval times, we want an entrance that is closed by default and can be quickly opened and closed to let people in. And, if possible cars and similar stuff too, but that would be a bonus not a necessity.

So the sideways rolling entry from the series is a good start, since it can be placed quickly and fulfills all the requirements. And if you park a car against it, it can probably hold against a pretty big pileup. But you have the risk of someone forgetting to lock it. And if you kill the zombies, you have a big pile of corpses and no guaranty that you got all of them, so moving them per hand would always carry a little risk.

So we use the protection of this gate to build a kind of portcullis. You'll have to build two big towerlike constructions as guiding rails, but you have some time. The grate itself should be heavy is best connected to some winding mechanism. This way it can not be raised by hand by a (human) attacker, but only from inside. This wind should be connected to a motor (a car could be modified for this) to raise the gate quickly. But there should still be a manual way to raise the gate, if you run out of gas or the the car breaks or something.
Important is, that the gate lowers itself after a short time, if not attended to, so it can not be left open by accident. To accomlish this, the motor should only run while a button is pressed (or something similar, depends on the resources available and the specific way the motor was implemented in the construction) and the gate must not have a mechanism to arrest it in an open position (like the medieval ones did).
The best position for the portcullis is directly behind the old gate, so we only lose a little space and can build it in the safety provided by the gate. After we constructed and tested the portcullis, we can deconstruct the old gate to save resources and speed up the process of opening up for incoming friends/allies.

Now we are safe against forgetting to lock the door or the guard getting killed while the door is open. But there is still the problem with possible pile-ups and also there can be an attack of humans, who use for example a truck as a ram. The answer to this is a moat and a drawbridge.
We need the bridge first, since we cannot pass the moat without it. The bridge itself can be prepared inside, but the hinges have to be build in place. Best position for this would be, where the old gate was.
Before we install the bride, we also have to start some more complex constructions: counterweights. We want to lower and raise the bridge quickly and it should be massive enough, so (best case scenario) even trucks can pass it without problems. That means we have a pretty heavy construction. So we also need a stable construction with thick steel chains and girders, that is able to support the weight. This has to be constructed inside and put up in a short amount of time, since putting it up probably makes a lot of noise, thus attracting zombies. But after we build it, we can start digging a moat.
We have to be careful while doing so, not to undermine our structures for the drawbridge (remember, that thing is heavy). The moat should be at least 2m (7 feet) deep, so the zombies can not climb out. Now, we have taken care of the pile-up problem and are save against attacks by ram-like constructions. Most zombies will fall into the moat and can be moistened with some oil or fuel and burned from a safe distance. A little smelly but save.
And a well balanced bridge can be quickly lowered and raised by hand.

Now there is only one thing left to do for a perfect defense: a second drawbridge. We construct a small island surrounded by a moat. One bridge connects it to the outside and one to the inside of out fortress. We sacrifice some space to do so, since the structure is probably to complex to be build outside the walls, be it will be worth it.
Now, if someone approaches the fortress, we lower the outer bridge and raise the portcullis. They can enter the island, the portcullis is lowered and the bridge raised.
If the approaching person or group is closely followed by zombies, we don't risk them getting inside. If the zombies were to close and catch the person/group, while entering, they still aren't inside the base and we can dispatch them from a distance (if they don't fall into the moat anyway). If the arrivals make it to the island without any zombies coming in, we can safely lower the inner bridge and let them in.
Also, there is the possibility of people arriving, we don't know (or trust) yet. We can let them get on the island, thus granting them asylum and some safety from the zombies without actually letting them inside.
Now we have an extremely safe gate and it would be a good time to expand the moat, so it reaches all around the fortress.

The result reminds a lot of medieval castles, but there are small differences. Mainly that the default state is not, ready to be closed, but closed, ready to be opened and closed again. And we can use motorized mechanisms for the portcullis.

$\begingroup$Good point. How do you secure the door for when you want to get in and out?$\endgroup$
– PatJFeb 24 '17 at 3:33

$\begingroup$I din't thought the part of people coming in, maybe finising the zombies on the door with fire, big blades swinging, or something else to kill them? But if your fortress is surrounded by zombies, how anyone can approach?$\endgroup$
– FernandoFeb 24 '17 at 13:42

A dumb idea or work around would be the trope of Razor floss. something like a super thin wire set into a door way such that as the pressure of the zombies mounted they just cut the front line in to bits. Your number 1 problem becoming dealing with body disposal.

$\begingroup$Like a zombie cheese cutter, I like it! Trouble is getting through bone. I think you'll wind up with the first rank of zombies sort of embedded in the wire, stopped from going further by their bones. Eventually the press behind them will break the wires.$\endgroup$
– SchwernFeb 21 '17 at 21:43

$\begingroup$the first layer of zombies will just rip down the wire when it hits their bones.$\endgroup$
– JohnFeb 22 '17 at 15:27

Zombies wouldn't clump as they are shown, and enough pressure to break a door is not achieved by a zombie horde even if they did, and even if it were, the first few zombies would be crushed and block the path from zombies entering, and even if that were not the case they're all funneling into the same passage making them easy targets since they have to come in 1 at a time.

$\begingroup$it only takes a few people to knock down an normal door.$\endgroup$
– JohnFeb 22 '17 at 15:25

$\begingroup$@John It takes 1 person to kick a door in. People shambling into a door cant do cuz the force is distributed over the full door and through the body of the corpse. you aren't going to get enough force to push through a door with any number of people just trying to walk through it. Because that force is countered by the bodies of the zombies. doors and such collapsing is due to people putting more force into moving forward or whereever rather than what shambling does.$\endgroup$
– DurakkenFeb 22 '17 at 16:50

1

$\begingroup$a bunch of people pushing together can bust open a normal door, it doesn't take much force to split a wooden door drame. Modern houses just are not built that study. The large surface of the door actually makes it easier to exert more force on the hinges and frame.$\endgroup$
– JohnFeb 22 '17 at 20:52

$\begingroup$@John Try it. Get 100 people and see if you can break down a door by walking into it it, not pushing but just walking slowly in the general direction of the door. I bet you'd find you can't.$\endgroup$
– DurakkenFeb 23 '17 at 5:56

2

$\begingroup$I worked demolition I have broken down many doors. you can break open a door with two decent sized people leaning on it. the door jamb is just a sliver op pine it does not take much to split it.$\endgroup$
– JohnFeb 23 '17 at 14:13

Redirect and mislead.

Build an outer wall, with an obvious and noisily entry gate; the zombies enter it, and are led into a kill zone (one or several big holes, spikes at the bottom), with a dead end. They get nowhere near the inner wall's gate.

Regular humans will enter/exit the outer wall by a concealed outer gate, not noticeable by the zombies (they're dumb).

$\begingroup$I did this on an old game called "Evil Genius" There was a great big steel locked door leading to a kill zone and my minion accessed my base from the unlocked wooden door next to it. All the enemy agents would leave the unlocked door alone and try and break into the locked one. If they got in, they got splattered.$\endgroup$
– ThorneMar 29 '17 at 3:56

Well I think a rather simple design might be to steal the idea of the Plow from above and simply make it the gate.

So instead of a straight across/up gate make it some version of a V type gate. You can construct the braces with a beam/bar fitting into the door panel and the ground.

Get adventurous and have a platform covering the opening at the top for easy pickings vs the Zombies.

Get even more adventurous and have a walkway up there that will channel the zombies to the end were they fall (get pushed) out. Maybe put a grinder were they fall.

Get even more adventurous and provide slots for pikes (as mentioned in another answer).

Should be relatively easy to open and close.

An attempt at drawing :)

########
# #
# #
# #
### ###
\/

Pressuring the doors from the outside push's then together and the individual door can be braced.

Platform on top of the V can be used to attack the zombies. Equaly any number of schemes for funneling the Zombies can be used here. Most notably would be steps up on both sides with a walk way away from the door to the outer edge which could simply be a drop to the ground, grinder, pit (hum Fire Pit?) etc. Regardless the spirit of this is to funnel the "horde" away from the gate in a constant flowing manner.

Several answers posted show gates that could survive any conceivable zombie attack. I wanted to mention the best defense would be one that stops the zombies before they ever reach the compound wall in the first place. My solution would be to build a secondary wall and audio trails to keep zombies away.

We know from earlier Walking Dead episodes that zombies are attracted to sound and light. None of the towns really use sound or light decoys to direct zombies away from a compound because the show uses the zombie horde attack as a plot device.

If this was a real life situation, building a secondary fence away from the compound would help direct the zombies to another location. If you build a series of audio devices along fence line, you create an audio trail to draw the zombies away from your compound area without directly interacting with them, saving you time and effort. This way zombies that are some distance away might be redirected and never approach your area in the first place. If they never enter your area, they never become a potential threat to your gate.

There is something that we know can take an awful lot of pressure. The breech of a cannon.

Make the door in a short cannon barrel as a breech, preferably with a diameter so that you can have trucks driving through.

You'll probably need a plate that can be pushed forward from the door to clear out zombies in the barrel. If fire destroys these zombies then a regular burst from a flamethrower from a hole in the top of the barrel followed by a push from the plate and the barrel is clean again.

The zombies will have to move the wall because they are not going to move the door unless it is opened from the inside.

Since this is the walking dead zombies we're designing this wall against all we would need to do is direct sounds inside the wall away from the outside of the wall - this sounds dumb and hard, but I think it's doable when you consider how sound is already being directed away from places the user is likely to be in some gaming desktops - and tie de-toothed walkers to the front entrance. This way the walkers won't be able to hear the humans inside the wall and they won't be able to tell the difference between the wall and de-fanged walkers.

The better question is why would you want to do this? none of the walls in the walking dead have ever failed because of walkers. They always fail because the people do something stupid in terms of maintenance with the wall. The little girl feeds the walkers rats at the prison and then someone drives a tank through the gate. The wall at Alexandria fell because someone didn't do anything about the building that could, and did, fall through the wall. The wall at the hilltop "broke" because humans on the outside broke it. These aren't things that repelling walkers from the gate is going to fix. If you want to fix this stuff then you should really just build bigger walls. Notice that no one has a secondary gate. No one has a second wall either. When you look at the walls in the walking dead they're just poorly designed for defensive purposes because they were put together in a rush to deal with walkers. All of the people with worse walls than the ones featured in the show currently have lost their settlements because the built garbage walls. Remember Noah's colony with the week 10ft wall? Compare that wall to the one at the hilltop.

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